This invention relates to a process for manufacturing thermosetting resin particles, particularly for powder coating use, having a narrow particle size distribution.
There are a number of reports and patent literature addressing the manufacture of micron size resin particles having a narrow particle size distribution. One of known methods for manufacturing such resin particles is the suspension polymerization method in which vinyl monomers containing an oil-soluble initiator are dispersed as oil droplets in an aqueous medium containing a stabilizer and then polymerized. However, this method gives resin particles having a wide particle distribution under normal stirring conditions. This is because polymer particles tend to adhere to reaction vessel walls or mixing propellers and the particle size distribution depends mainly upon the incidence of agglomerating and splitting of monomer droplets. In order to prevent these phenomena from happening, several methods have been proposed including the use of viscous monomer droplets in suspension polymerization by dissolving a portion of polymer in the monomer or partly polymerizing the monomer in bulk prior to the suspension polymerization. Other methods include the use of strongly surface active stabilizer or water-insoluble inorganic particles in the dispersing medium. These methods are generally effective to decrease the proportion of coarse particles because of improved mixing efficiency and decreased agglomeration but they are not effective to decrease the proportion of fine particles. Consequently, the particle size distribution represented by the ratio of weight average particle size to number average particle size can be improved only slightly by these methods.
The seed polymerization and swelling method disclosed in JP-A-58106554 can give linear or crosslinked vinyl polymer particles having a very narrow particle size distribution in which the weight average particle size nearly equals the number average particle size. Unfortunately, this method requires a number of steps for growing polymer particles, making it unsuitable for a large scale industrial application. In addition, it cannot be applied to the manufacture of polymer particles including foreign matter such as pigments.
JP-A-03200976 discloses a method for manufacturing colored or pigmented polymer particles in which monomers are polymerized in a dispersion in nonaqueous systems or solvent-water mixture systems. Because a large quantity of solvent is used, this method suffers from safety and environmental problems in handling, recovering or otherwise processing used solvents.
All of the methods discussed supra utilize a radical polymerization of vinyl monomers and produce only vinyl based polymer particles having limited area of use. Furthermore, they are not applicable to the manufacture of particles including those substances having adverse effects on the polymerization reaction, for example, some dyes and pigments or UV absorbers and antioxidants abundantly used in automobile finishes.
Recently much interest has been focussed on powder coating in finishing automobile bodies and parts, household electrical apparatuses, building materials and the like for the purposes of eliminating the emission of organic solvents to the environment. Powder coating compositions are generally produced by blending a binder resin with a crosslinker and optionally other additives such as pigments, kneading the mixture under heat to make a molten mass, pulverizing the mass and then classifying pulverized particles. The resulting particles are applied on a substrate using electrostatic spray coating, fluidized bed coating or other methods to form a film, and then baking the film. However, most of the prior art powder coating compositions have certain defects. Since they tend to cause premature curing before use, chemicals or substances which react at a relatively low temperature cannot be added. Because the weight average particle size of conventional powder coating compositions normally lie at around 30 .mu.m, they are not satisfactory in terms of smoothness, luster and other aesthetic properties of finished films. Attempts have been made to produce finer particles having a weight average particle size of 10 .mu.m or less by means of jet mills or other air stream mills. This approach is found effective to obtain much smoother and thinner films than the films of conventional powder coating compositions. On the other hand, this powder coating composition comprises not only a large portion of microfine particles but also particles of irregular configuration. This makes the powder less free-flowing and increases the incidence of clogging of pneumatic conveyer pipes. Furthermore, it is conventional practice to recover and reuse excessively applied powder in powder coating technology. If the proportion of microfine particles increases in the recovered powder, not only will pneumatic transportation become more difficult but also deposition efficiency of the powder onto the substrate will be greatly impaired.
Therefore, a need exists for a process for manufacturing powder coating compositions free from above-discussed problems.